1. Field of the Invention
The present invention relates to a manufacturing method of a silicon wafer for semiconductor devices and, more specifically, to a silicon wafer manufacturing method in which silicon wafers are subjected to a heat treatment in an atmosphere containing hydrogen.
2. Description of the Related Art
In the silicon wafer manufacture, it is a conventional technique to reduce the oxygen concentration of an active layer close to the wafer surface where semiconductor devices are to be formed by performing a heat treatment in an hydrogen atmosphere or a mixed atmosphere of a hydrogen gas and an inert gas (in the following, both atmospheres are referred to as a hydrogen-gas containing atmosphere), and to thereby eliminate almost completely precipitation defects due to oxygen precipitation, such as BMD. (Refer to, for instance, Nikkei Microdevices, June 1993, pp. 46-55.)
In performing a heat treatment on silicon wafers (in the following, also simply called wafers) in a hydrogen-gas containing atmosphere, the closest attention is paid to avoid hydrogen explosion. For example, a heat treatment is performed according to the following procedure.
Step 1: The inside of a furnace is opened to the air.
Step 2: Wafers are introduced into the furnace.
Step 3: After the furnace is closed, the intrafurnace atmosphere is replaced by a nitrogen gas.
Step 4: After oxygen is completely eliminated from the furnace, a hydrogen gas is introduced into the furnace and substituted for the nitrogen gas.
Step 5: A heat treatment is started.
Step 6: After completion of the heat treatment, the intrafurnace atmosphere is replaced by a nitrogen gas.
Step 7: The inside of the furnace is opened to the air, and the wafers are taken out of the furnace.
Where wafers are consecutively subjected to a heat treatment in a hydrogen-gas containing atmosphere as a batch process, step 2 is performed after step 7.
Steps 3 and 6 for replacing the intrafurnace atmosphere with a nitrogen gas are usually performed at 700-800.degree. C., and steps 2 and 7 for inputting/outputting wafers are usually performed approximately at the above temperature. In recent years, attempts of increasing the above temperature have been made to improve the production efficiency.
However, in wafers as subjected to a heat treatment in a hydrogen-gas containing atmosphere, a surface oxide film has been removed to expose the silicon surface. Therefore, there is a problem that if a nitrogen substitution gas is used at a high temperature (more than 800.degree. C.), it easily reacts with silicon of the wafer surface, to form a nitride, in which case wafers become defective and cannot be used anymore. Even in a temperature range of 700-800.degree. C., a nitride film is formed on part of wafers or wafer surfaces are roughened.
Wafers as subjected to a heat treatment in a hydrogen-gas containing atmosphere have very active surfaces, and are therefore prone to be polluted with metal impurities etc. Where a nitrogen substitution gas is used, the surface activity can be reduced by formation of a nitride film, which, however, may cause defective wafers and is therefore not preferable.
When some abnormality in a heat treatment requires urgent discharge of a hydrogen-containing gas from a furnace, a nitride film is formed on every wafer if a nitrogen gas is used for that purpose. Therefore, all the wafers become defective.
It has been attempted in the conventional method to use an argon gas instead of a nitrogen gas. However, due to the problems exemplified below, usually an argon gas is not used positively as a substitution gas. That is, an argon gas available on the market is less pure and more expensive than a nitrogen gas. Experiments have revealed that an argon gas likely makes a wafer surface cloudy, though detailed reasoning has not been made. (Refer to, for instance, "Collection of Safety Data of Semiconductor Process Gases," coauthored by the Special Gas Industrial Society and SEMI Japan, 1991, pp. 2. Further, an argon gas likely roughens a wafer surface.
In summary, the conventional method of using a nitrogen substitution gas is associated with the technical problems that wafers become defective at a high rate, and that the wafer input/output temperature cannot be set high.
While the use of an argon gas can prevent formation of a nitride film, it causes other problems such as surface clouding and roughening.